Autism spectrum disorder (ASD) is characterized by core social impairments and affects 1 in 68 US children, but remains poorly understood. ASD is currently diagnosed on the basis of behavioral criteria because no robust biomarkers have been identified, nor are there any pharmacotherapies that effectively treat the social features of ASD. Biomarkers improve our ability to make accurate diagnoses and provide biological targets for drug development and testing. Two promising biomarkers of ASD are the closely related neuropeptides oxytocin (OXT) and arginine-vasopressin (AVP). OXT and AVP are critical for mammalian social functioning, and experimental disruption of these signaling pathways produces social deficits in animal models with relevance to ASD. In parallel with these preclinical studies, diminished blood OXT and AVP concentrations have been associated with social impairments in children with ASD, suggesting that both neuropeptides hold therapeutic promise for treating this disorder. Furthermore, human genetic studies have shown an association between common genetic variants of both the OXT receptor (OXTR) and AVP receptor (AVPR) v1a genes and ASD. A complementary regulatory mechanism to direct alteration of the DNA sequence is epigenetic modification of DNA, which may have extremely important scientific implications for understanding ASD disease biology. One epigenetic process, DNA methylation, influences gene expression and, at the extreme, can greatly diminish or even silence gene expression. Our team is now conducting OXT and AVP treatment trials in children with ASD. Treatment response is promising, but variable, and this may be due to variation in the availability of OXTR and AVPR v1a on which the drugs can effectively act to enhance social functioning in each patient. Despite the obvious need, no prior research has tested whether variation in DNA methylation of the OXTR and/or AVPR v1a genes has social, diagnostic, and/or treatment implications for individuals with ASD. These critical gaps in knowledge will be addressed in this highly innovative R21 research project. Our initial project goals are: (1) To test whether OXTR and/or AVPR v1a gene methylation differs between, and predicts disorder classification of, children with and without ASD; (2) To test whether OXTR and/or AVPR v1a gene methylation predicts social functioning and diagnostic scores; and (3) To identify, on an exploratory basis, additional genes that may be epigenetically regulated in children with ASD using genome-wide discovery methods. We will next confirm promising findings from this first study in a validation cohort. We will also test in this cohort whether variation in DNA methylation and expression of previously identified genes predicts treatment response in children with ASD who have completed our OXT and AVP clinical trials. This research could lead to the implementation of earlier and more accurate diagnostic methods, discovery of novel biological targets, and development of the first personalized therapeutics to treat the social impairments in ASD.